One of the most urgent environmental problems facing expert circles is the disposal of wastes, including those made of plastic. Heretofore, such materials were mixed with other wastes and stored in landfills, but this has proven to be a questionable procedure because it does not take into account the long-term effect on ground-water and soil. Attempts are made to avoid such environmental pollution by storage in special landfills but, because there are only a limited number of appropriate discharge sites, disposal of the wastes in an environmentally neutral manner is actually only being shifted to the future.
Therefore, many attempts have been made recently to develop processes for reprocessing such wastes. They do not have the sole aim of protecting the environment, but frequently include the recovery of utilizable products from materials which are no longer useful for their original designated purpose.
The reprocessing of used or off-specification plastics to give reusable original material fails in most cases because the wastes contain plastics of different material compositions. It is easily seen that such mixtures generally cannot be reprocessed to give the original material or an equivalent thereof. The separation of the mixtures into their individual components fails because of the difficulty of identifying them. Moreover, only in exceptional cases can starting material of the original quality be recovered from wastes of even identical plastics, since the necessary chemical and/or physical treatment steps change the molecular structure of the polymers and thus their properties.
Plastic wastes can be incinerated only under conditions which ensure that no pollutants pass into the atmosphere. This prerequirement is satisfied only in exceptional cases, since the plastics frequently include chlorine-, sulfur-, or nitrogen-containing constituents, as well as heavy metals, all of which lead to undesirable combustion products. Dust removal and flue gas scrubbing (and sometimes special combustion apparatus) are then unavoidable. Transport and metering problems can arise if the wastes also contain non-combustible and non-melting foreign materials. Moreover, economic reasons argue against burning high-grade processing products of petrochemical raw materials, just as they argue against burning their raw materials, i.e. petroleum and petroleum products.
Instead of burning them, plastics which are no longer utilizable have also been thermally cracked. The processes developed therefor take many forms. Thus, by the breakdown of polyethylene at 400.degree. to 450.degree. C., a gasoline/kerosene mixture is obtained (C.A. Vol. 76, 1972, 158024 q). This process can also be carried out in the presence of nickel catalysts (Chem. Ind. XXIII, 1971, 630). The cracking of carbon-containing organic wastes of synthetic or primarily synthetic origin is carried out by the process of EP-A-291 698 under hydrogenating conditions and predominantly yields hydrocarbon fractions in the gasoline and middle oil (diesel oil) boiling range. Plastic and rubber wastes are thermally cracked by the process described in DE-C-2 205 001 at 250.degree. to 450.degree. C. in the presence of an auxiliary phase liquid at the reaction temperature. Over 90% liquid hydrocarbons are produced and, in lesser amounts, soot.
An obvious desirable aim of the thermal treatment is the conversion of the plastics into liquid fuels, which can easily be transported, metered, and homogeneously distributed in the combustion air to ensure smoke-free and soot-free combustion. Prior use of the hydrocarbons, e.g. as solvents, extractants, or as cleaning agents is not excluded in this case.
Important disadvantages of the known processes are the requirement to very substantially degrade the plastics, the necessity of maintaining corresponding temperatures and residence times, and the need for complex separation of the solids, such as inorganic or organic pigments, opacifiers, and fillers, which are frequently present in the plastics.